It is known that one of the factors limiting observation systems using non-geostationary moving satellites is the capacity of the downlink of a moving satellite to downlink all the images taken by the satellite to the receiving ground station or stations when said moving satellite is passing over said receiving ground station with the latter geometrically visible from said satellite.
In recent decades, in order to increase the downlink capacity of each of the moving satellites, there has been an increase in the frequencies used for downlinks in satellite observation systems, which are now moving towards the X and Ka bands, in order to be able to convey and downlink increasing amounts of image data during the given time period of the satellite's pass when it is visible from the station.
Existing projects aim to continue this move towards optical frequencies, which would allow yet more telemetry data to be downloaded in the given time period during which the satellite is flying over the receiving ground station.
However, starting in the very high frequency radio bands, i.e. above the UHF band, attenuation due to the presence of clouds in the atmosphere significantly limits throughput, and this attenuation becomes prohibitive for optical links.
One solution that makes it possible to use higher frequencies on the downlink consists in using a communication relay platform, installed on board an aircraft, for example a drone or balloon, traveling at high altitude above the clouds, helping eliminate the prohibitive attenuation of radio signals caused by the clouds, which then only affect retransmission between the communication platform and the receiving ground station, which retransmission takes place over a much shorter distance (a few to several tens of kilometers rather than several hundred to thousands of kilometers) and in a location allowing the use of specific allocations of transmission frequencies and bands, with the further major advantage of being able to be permanent.
Despite the existence of regulatory requirements for the bands that can be used between high-altitude relays and the ground, and although these bands are not necessarily advantageous in terms of accessible throughput, these usable bands are less congested than the bands allocated to satellites.
Using a high-altitude relay as the relay between the ground and moving satellites is highly beneficial because, since the relay is above the clouds, an optical downlink between the moving satellite and the high-altitude relay platform allows high-throughput communications with 100% availability during the satellite's pass when it is visible from the relay, typically a few minutes to approximately ten minutes in the case of low orbit satellites. The second downlink between the relay platform and the receiving ground station can be a radio frequency downlink in a frequency band less sensitive to clouds and rainfall, with a smaller throughput compensated for by permanent visibility. Whereas a moving observation satellite spends approximately 40 minutes per day visible to a ground station, the high-altitude relay allows an improvement of a factor of more than 30 in terms of the throughput required from the relay station to the ground.
Patent application U.S. Pat. No. 6,151,308 A describes a satellite system using high-altitude relay platforms that make it possible to define communication cells more precisely, and avoid obstructions in satellite links between terminals and satellites moving in a low orbit, in particular when terminals are located in an urban environment, thus allowing a greater number of terminals to access the network of the communications system. Patent application U.S. Pat. No. 6,151,308 A describes an advantageous use of the satellite system in the context of an observation mission that makes it possible to obtain finer and higher resolution images by installing the observation instruments on high-altitude relay platforms rather than moving satellites. Document U.S. Pat. No. 6,151,308 A does not describe the use of a communication relay platform to help return voluminous telemetry data from moving observation satellites and does not describe how the security of transmissions is managed at the satellite observation system, and in particular at the communication relay platform, in the case of relays with a plurality of satellites and/or multiple receiving ground stations.
A first technical problem is that of providing an architecture for integrating security functions, at the satellite observation system and in particular at the communication relay platform, that simplifies how the security of transmissions is managed in the case of a plurality of satellites and/or multiple receiving ground stations.
A second technical problem is that of increasing the availability and capacity of the downlink from the relay platform to the ground station in order to overcome a prohibitive drop in throughput or an absence of downlink between the relay platform and the ground station caused, for example, by interference from clouds that are highly absorbent with regard to radiation.